- “To my mutant brothers and sisters out there, I say this: No more hiding. No more suffering. You have lived in the shadows in shame and fear for far too long. Come out. Join me. Fight together in a brotherhood of our kind. A new tomorrow, that starts today.”
The Brotherhood of Mutants is/was a terrorist organization fighting for mutant supremacy over humans. They were founded and led by Magneto.
After the death of Sebastian Shaw during the Cuban Missile Crisis, Magneto forms the Brotherhood of Mutants with the remnants of Shaw's Hellfire Club. The original team consisted of Magneto, Mystique, Riptide, Azazel, and Angel Salvadore, and, soon after, they staged a raid on the CIA's headquarters to release Emma Frost from her prison and recruit her into their ranks.
X-Men: Days of Future Past Prologue
Decades later, the Brotherhood consisted of Magneto, Mystique, Toad, and Sabretooth. They planned on using Rogue to power a conversion machine that would turn humans into mutants. In the resulting fight, Toad was electrocuted by Storm, Sabretooth was knocked out from the Statue of Liberty by Cyclops, and Magneto was defeated and incarcerated in a specialized Plastic Prison designed by William Stryker to negate his powers. However, Mystique survives the battle and goes into hiding.
Mystique helped Magneto escape from his plastic prison, and the two work together with the X-Men to stop Stryker's plan to initiate a mutant genocide. After foiling Stryker's plan, Magneto uses Stryker's Cerebro to try and kill all of the humans instead, but this plan is in turn foiled by the X-Men. Pyro betrays the X-Men and joins the Brotherhood with Magneto and Mystique while they escape in a helicopter.
Following the announcement by Worthington Labs of a supposed mutant cure, Magneto expands the Brotherhood with the inclusion of the Omegas to stop the unveiling, eventually leading to a battle at Alcatraz Island in which most of its members are either killed or de-powered. The Brotherhood was disbanded indefinitely after Magneto lost his powers.
Angered at Charles over the mutant's choice to stay in hiding, Erik would point out the deaths of Angel, Azazel, and Emma to show the atrocities that had been committed while he had been gone. Later, while trying to assassinate Nixon, Erik broadcasted himself across the entire world, urging mutants to come out of hiding and stand with him in a "brotherhood" where they would make a better tomorrow by destroying the "inferior" human race.